1. Field of the Invention
The present invention relates to acid-in-oil emulsions and methods of stimulating the production of hydrocarbons from subterranean formations using such emulsions.
2. Description of the Prior Art
The use of aqueous acid solutions to stimulate the production of hydrocarbons from subterranean formations having naturally low permeabilities is well known. Typically, an aqueous acid solution is pumped through the well bore into the formation where it reacts with materials in the formation to increase the permeability thereof. The increased permeability brings about an increase in the flow of hydrocarbons from the formation into the well bore.
When a formation having naturally low permeability is contacted with an aqueous acid solution, the acid solution rapidly diffuses and reacts with the materials making up the formation. As a result, the delivery of the acid solution deeply and uniformly into the formation before it spends is often difficult or impossible. In as much as the flow of hydrocarbons in formations having low permeabilities is only increased in those areas to which unspent acid is delivered, any increase in hydrocarbon production is proportional to the distance from the well bore reached by unspent acid.
A variety of acid compositions and methods have been developed whereby the rates of acid diffusion and acid spending are retarded and unspent acid flows greater distances within a formation being treated. A particularly advantageous such composition is an acid-in-oil emulsion which is retarded as a result of oil being the external phase and acid the internal phase. The external oil phase provides a barrier to the diffusion and spending of the acid allowing unspent acid to be delivered deeply and uniformly into a formation. However, while acid-in-oil emulsions have been effective in acidizing some formations, they have been relatively ineffective in other formations having naturally low permeabilities. A principle reason for such ineffectiveness is that acid-in-oil emulsions have relatively high viscosities which often prevent them from being pumped through a well bore and into a formation to be treated at a high enough rate. That is, the high friction pressure of a viscous emulsion often increases the injection pressure whereby the desired rate can not be pumped without exceeding the pressure limits of tubular goods and other equipment. Another problem in the use of prior acid-in-oil emulsions relates to the reactivity of the acid in low temperature formations, i.e., temperatures in the range of from about 50.degree. F. to about 150.degree. F. At such low temperatures, the acid generally has a low reactivity with formation materials in the presence of oil which prevents the acid from substantially increasing the permeability of the formation.
Thus, there is a need for improved well acidizing compositions and methods whereby the acidizing compositions have low viscosities, are retarded so that unspent acid can be efficiently delivered deeply and uniformly into the formation and the acid reacts with the materials making up the formation in the presence of oil at a rate sufficient to effectively increase the permeability of the formation and the production of hydrocarbons therefrom.